The present invention relates to a semiconductor device and a method of making the same, and more particularly to a metal oxide semiconductor transistor and a method of making the same which can improve the junction leakage current and the insulation characteristic by floating a source region and a drain region.
FIGS. 1A to 1F are sectional views showing a method for making a general metal oxide semiconductor (MOS) transistor having a n-type.
Referring to FIG. 1A, a pad oxide 13 and a pad nitride film 15 and a p type of substrate 11, in this order.
A photoresist 17 is coated on the pad nitride film 15 and then patterned, to define active regions 19 and field regions 20.
Referring to FIG. 1B, the pad nitride film 15 and the pad oxide film 13 are etched in this order using the patterned photo resist as an etch mask, to expose the silicon substrate 11 corresponding to the field regions and the patterned photo resist is then removed.
Referring to FIG. 1C, an oxide film is grown on the exposed silicon substrate 11 using a field oxidation process, thereby causing field oxide films 21 for device seperation to be formed thickly.
Referring to FIG. 1d, the pad oxide film 13 for field oxidation and the pad nitride film 15 are all removed and impurity ions are then implanted into the silicon substrate 11 to control the threshold voltage V.sub.th. Referring to FIG. 1E, a thin oxide film 23 is grown on the silicon substrate 11 and a polysilicon film 25 is then deposited on the thin oxide film 23.
Thereafter, the thin oxide film 23 and the polysilicon film 25 are patterned to form gate oxide films and gate electrodes.
As shown in FIG. 1F, n-type of impurity ions are implanted in the silicon substrate 11 using the gate electrode as an ion-implantation mask, to form source regions 27 and drain regions 27.
However, since the MOS transistor shown in FIG. 1F does merely use the field oxide films 21 as the seperation regions for electrically seperating devices, there is a disadvantage in that the seperation region has a great loss in plane.
Since n-type of impurity diffusion regions serving as the source regions 27 and the drain regions 27 are directly formed on the p-type of silicon substrate 11, thereby resulting PN junction to be formed, there is a disadvantage in that the PN junction served as several leakage paths.
As shown in FIG. 1D. since the field oxide film 21 is also thickly formed, thereby causing steps between the field oxide 21 and the silicon substrate 11 to be formed considerably highly, there is a disadvantage in that it is difficult to perform a following process due to the steps.
The demand to the reduction in the size of device has reduced the size of respective transistor formed in a large scale integrated(LSI).
However, the regions between transistors has also been reduced in size due to the reduction in size of respective transistor.
In a high-integrated MOS transistor which used a thick oxide films as the device seperation regions between transistors, there is a disadvantage in that the punch-through phenomenon may be occurred between transistors as the regions between transistors become still more close.
A method for solving the punch-trough has been suggested.
This method dopes impurity ions into a silicon substrate with a high concentration, such that the surface concentration of silicon substrate between transistor becomes high.
According to this method, however, there is a disadvantage in that the junction capacitance is increased and the increasing of junction capacitance does moreover have a bad influence on the operation of device in high speed as the concentration of substrate is increased.
So as to solve this problem, there has been proposed a semiconductor device such as silicon on insulator (SOI) in which an insulation film is formed on a silicon substrate to insulate the active regions from the silicon substrate electrically.
FIG. 2 is to show a sectional view of a general SOI semiconductor device.
Reference number 31 denotes a silicon substrate, 32 denotes a buried oxide film for insulating a p-type of silicon active region 33 from the silicon substrate electrically, 34 and 35 denotes a n-type of source region and n-type of drain region respectively, 36 denotes a thin oxide film which is a gate insulation film and 37 denotes a gate electrode.
According to the above MOS transistor, its active regions 33 are depleted in accordance with a voltage applied to the gate electrode 37, it is possible not only to restrain the electric field of drain applied to between the drain region 35 and the active region 33 but also to restrain the short channel effect of threshold voltage.
If the thickness of the buried oxide film 32 beneath the drain region 35 is increased, it is also possible to reduce the capacitance of parasitic junction.
Therefore, the SOI MOS transistor has an advantage capable of obtaining the degree of high integration and the operation characteristic of high speed.
However, if the thickness of buried oxide film 32 beneath the active layer 33 is too much thick, the electric field of drain region has a bad influence adversely on the distribution of electric field at the active region 33, thereby causing the short channel effect to be increased.
On the other hand, if the thickness of buried oxide film 32 beneath the active layer 33 becomes thinner than that of FIG. 2, it is possible to restrain the short channel effect. But the parasitic capacitance is increased in accordance with the reduction of thickness of buried oxide film 32 beneath the drain region 35.
Therefore, it is impossible to obtain the operation-characteristic of high speed.